This disclosure relates to organic light emitting devices (OLEDs), and finds particular application in conjunction with display devices utilizing electroluminescence technology.
Disclosed herein are organic electroluminescent (EL) devices and, more specifically, organic EL devices with a number of excellent performance characteristics inclusive of the enablement of blue emitting EL devices. These devices contain luminescent components or a luminescent component with excellent high thermal stability, film forming characteristics and intense blue fluorescence. Organic EL devices are desired that are capable of providing uniform luminescence, saturated color especially in the blue regions of the visible spectrum, and low driving voltages. The organic EL devices disclosed herein enable the above characteristics. These devices contain organic luminescent materials or light emitting components comprised of fluorescent 1,1′-binaphthyl derivatives. These devices can be selected for use in flat-panel emissive display technologies, including TV screens, computer screens, and the like.
A simple organic EL device can be comprised of a layer of an organic luminescent material conductively sandwiched between an anode, typically comprised of a transparent conductor, such as indium tin oxide, and a cathode, such as low work function metal like magnesium, calcium, aluminum, or the alloys thereof with other metals. The EL device functions on the principle that under an electric field, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the luminescent layer and undergo recombination to form excitonic states which subsequently emit light.
A number of conventional organic EL devices have been prepared from a laminate of an organic luminescent material and electrodes of opposite polarity. These devices include a single crystal material, such as single crystal anthracene, as the luminescent substance as described, for example, in U.S. Pat. No. 3,530,325. However, these devices require excitation voltages on the order of 100 volts or greater. Subsequent modifications of this organic EL device structure through incorporation of additional layers, such as certain charge injecting and charge transporting layers, may result in performance improvements.
An organic EL device with a multilayer structure can be formed as a dual layer structure comprising one organic layer adjacent to the anode supporting hole transport, and another organic layer adjacent to the cathode supporting electron transport and acting as the organic luminescent zone of the device. Another alternate device configuration is comprised of three separate layers, a hole transport layer, a luminescent layer, and an electron transport layer, which layers are laminated in sequence and are sandwiched between an anode and a cathode. Optionally, a fluorescent dopant material can be added to the emission zone or layer whereby the recombination of charges results in the excitation of the fluorescent material.
In U.S. Pat. No. 4,539,507 there is disclosed an EL device formed of a conductive glass transparent anode, a hole transporting layer of 1,1-bis(4-p-tolylaminophenyl)cyclohexane, an electron transporting layer of 4,4′-bis(5,7-di-tert-pentyl-2-benzoxzolyl)stilbene, and an indium cathode.
U.S. Pat. No. 4,720,432 discloses an organic EL device comprising a dual-layer hole injecting and transporting zone, one layer being comprised of porphyrinic compounds supporting hole injection and the other layer being comprised of aromatic tertiary amine compounds supporting hole transport.
U.S. Pat. No. 4,769,292 discloses an EL device employing a luminescent zone comprised of an organic host material capable of sustaining hole-electron recombination and a fluorescent dye material capable of emitting light in response to energy released by hole-electron recombination. A preferred host material is an aluminum complex of 8-hydroxyquinoline, namely tris(8-hydroxyquinolinate)aluminum.
While recent progress in organic EL research has elevated the potential of organic EL devices for widespread applications, the performance levels of current available devices, especially with respect to blue emission, may still be below expectations. Further, for visual display applications, organic luminescent materials should provide a satisfactory color in the visible spectrum, normally with emission maxima at about 460, 550 and 630 nanometers (nm) for blue, green and red. Alternatively, organic EL devices may comprise a light-emitting layer which is comprised of a host material doped with a guest fluorescent material that is responsible for color emission. For efficient down-shifting of EL emission wavelength in the host-guest emitting layer, it is highly desirable that the host material should fluorescence in the blue or shorter wavelength region.
In most conventional organic EL devices, the luminescent zone or layer is formed of a green-emitting luminophor of tris(8-hydroxyquinolinate)aluminum with certain fluorescent materials. U.S. Pat. No. 5,409,783 further discloses a red-emitting organic EL device by doping the tris(8-hydroxyquinolinate)aluminum layer with a red fluorescent dye. However, up-shifting of the tris(8-hydroxyquinolinate)aluminum emission to blue region is believed to be highly inefficient. Although there have been several disclosures describing blue-emitting organic EL devices, for example in U.S. Pat. Nos. 5,151,629 and 5,516,577, their performance characteristics still possess many disadvantages such as poor emission hue, high operation voltages, low luminance, and poor operation stability.
Accordingly, there continues to be a need for improved luminescent compositions for organic EL devices, which may be vacuum evaporable and form thin films with excellent thermal stability. There is also a need for luminescent compositions which are capable of providing uniform and satisfactory emission in the blue region of the light spectrum. In particular, there is a need for efficient blue luminescent materials for organic EL devices, which may optionally be doped with a fluorescent or phosphorescent dye. Further there is also a need for luminescent compositions which can enhance the charge transporting characteristics, thus lowering device driving voltages.
Therefore, a primary feature of the present disclosure is to provide new luminescent materials comprised of certain fluorescent 1,1′-binaphthyl derivatives which can provide improved and excellent emission characteristics particularly in the blue region, such as a saturated blue color and a narrow emission spectrum.
The present exemplary embodiments contemplate a new class of blue light emitting materials which overcomes the above-referenced problems and others.